For safety reasons, it is important that railroad rails are inspected regularly for the presence of flaws or defects. A flaw or defect may be an existing small crack, or a location where a crack could arise. Cracks have the tendency to grow, and a broken rail may have catastropic consequences, so it is important to detect potential crack sites as early as possible. Many cracks are rarely visible from the outside; if they are, the rail is probably completely broken, and this is a situation that is to be avoided. Hence, a technology is required that is capable of detecting flaws in the rail.
Such technology is ultrasonic measurement. Briefly said, an ultrasonic pulse is coupled into the rail, and the reflection of this pulse is captured. The pulse reflects from material surfaces, such as the outside rail surface but also the internal surface of a flaw. Thus, the reflection pattern in the case of a flaw differs from the reflection pattern in the case of an undisturbed rail. However, although the measurements could be performed stationary from a technical point of view, this is not desirable from a practical point of view, because the railroad should remain open for railroad traffic. Therefore, mobile systems have been developed that include a railroad vehicle carrying a mobile ultrasonic transducer.
Such mobile systems, however, introduce the complication that the ultrasonic transmitter and the ultrasonic receiver are moving with respect to the rail under inspection. One aspect of this complication is that it is more complicated to achieve a good signal coupling between rail and transducer. Another aspect of this complication is that the pulse measurement itself requires some measuring time, basically caused by the travelling time of the pulse from the ultrasonic transmitter to the reflection surface and back to the ultrasonic receiver. This sets restrictions on the minimum repetition frequency of the measurements. On the other hand, the repetition frequency of the measurements, or better: the time period between successive measurements, determines the spatial distance between the investigated rail locations, indicated as “inspection pitch”. With a certain maximum requirement for the inspection pitch (i.e. the pitch should be a certain value or shorter), a certain maximum operational speed for the inspection vehicle results (i.e. the operational speed can not be higher than a certain value). Given the fact that the railroad tends to be used more and more intensely, with regular trains driving faster and/or at closer mutual distance, it is desirable that the maximum operational speed for the inspection vehicle is as high as possible. If the inspection vehicles are too slow, they either disrupt regular service, or inspection can be done only during the night when traffic is reduced.
Mobile ultrasonic rail inspection systems can be distinguished in two basically different types. Both types require a liquid coupling substance between the ultrasonic transducer and the rail. A first type of inspection system is indicated as a contact-type system: in these systems, the transducer is held at a small distance from the rail, with a thin film of coupling liquid in between. A drawback of this type of system is the relatively large consumption of coupling liquid. This drawback is avoided in the second type of inspection system, which is indicated as a wheel-type system (or a “rotating” rail riding system): in these systems, the transducer is positioned inside a wheel-like container filled with coupling liquid and riding on the rail, which wheel-like container has a flexible wall that is able to conform to the shape of the rail head.
In practical circumstances, with an inspection pitch of about 3 mm, the maximum operational speed for the inspection vehicle is about 72 km/h for the contact-type system and about 37 km/h for the wheel-type system.